The invention is based on an electromagnet as generically defined by the preamble to claim 1.
In automatic transmissions in motor vehicles, for controlling the transmission actuators embodied as electromagnetically actuated pressure regulating valves are used, in which an electromagnet generates a magnetic force for performing a hydraulic control function. The dependency between the exciter current supplied to the magnet coil and the regulating pressure controlled is described by the characteristic curve of the pressure regulating valve. The characteristic curve has a steady course and is specified and forms the basis for the application in the motor vehicle. One essential characteristic of the characteristic curve is its slope. This slope is determined by the properties of the electromagnet. Fluctuations in the production tolerances of the individual parts of the electromagnet, however, affect the magnetic flux and thus the magnetic force, so that in the characteristic curves of identical actuators of the same production series, major fluctuations in the slope of the characteristic curves can occur.
In a known electromagnetic final control element with a proportional magnet for actuating proportional valves (European Patent Disclosure EP 0 464 370 B1), to eliminate the adverse effect of production variations and differences in permeability of the magnet material on the magnetic force of the proportional magnet and thus on the slope of the valve characteristic curve, technical measures have been taken for varying the magnetic resistance in the magnetic circuit within limits after the assembly of the electromagnet and thus to be able to calibrate electromagnets from the same production series retroactively with respect to their magnetic force. To that end, a displaceable adjusting sleeve of magnetically conductive material is slipped onto the end of a pressure tube, which receives the movable armature and forms part of the ferromagnetic circuit of the electromagnet. By displacing the adjusting sleeve, the overlap between the adjusting sleeve and the pressure tube, and thus the magnetic resistance that counteracts the magnetic flux, can be varied. The amount of the overlap determines the magnitude of the magnetic transitional resistance and thus affects the total resistance. If the overlap is zero or negative, major scattered fluxes occur, and the magnetic force engaging the armature drops considerably.
The electromagnet of the invention having the characteristics of claim 1 has the advantage that to create the capability of calibration for retroactive variation of the magnetic force intrinsic to the completed electromagnet, only minor structural changes to the magnet coil must be provided, while no comparatively major structural changes need to be made in the ferromagnetic circuit of the electromagnet. By welding at least two stanchions, each surrounded by one winding of the exciter winding, with the respective winding portion surrounding the stanchions, the part of the exciter winding located between the two winding portions welded to the stanchions is short-circuited, so that because of the reduced number of windings in the exciter winding, the magnetic force drops. Depending on the number of windings located between two welded-on stanchions, the magnetic force can be decreased to a greater or lesser extent. Since there are a plurality of stanchions and the winding packets located between the stanchions can be embodied with different numbers of windings, it is possible to reduce the magnetic force of the electromagnet with high precision by melting the applicable stanchions in such a way that they weld to the winding wire.
In order also to compensate for production variations that result in a lesser magnetic force than what is demanded, the exciter winding is embodied with a number of windings that is greater than the number of windings required for a magnetic force calculated in advance from the structural design of the electromagnet. Next, on the completed electromagnet, by intentional welding of the appropriate stanchions to the exciter winding, the requisite magnetic force is established. The welding is done by means of a laser beam. In the process, the stanchions melt open and produce a sufficiently large molten volume for secure welding.
By the provisions recited in the other claims, advantageous refinements of and improvements to the electromagnet defined by claim 1 are possible.
In one advantageous embodiment of the invention, one of the stanchions, preferably the stanchion around which the winding wire end is wrapped, is already welded to the winding wire end during the production of the electromagnet, while at least a further one of the other stanchions is not melted, for the sake of calibrating the magnetic force, until after the electromagnet has been completed. This shortens the calibration time, since for calibration purposes one fewer stanchion has to be melted.
In an advantageous embodiment of the invention, the electrical connection between the stanchions is produced by a rib of electrically conductive material, as a result of which the stanchions are combined into a one-piece stanchion strip. Such a stanchion strip can easily be secured to the coil body of the magnet coil, for instance by providing that the rib of the stanchion strip is injection-molded jointly in the production process for the coil body.
In a preferred embodiment of the invention, the rib, in a yarn-reel-like embodiment of the coil body that receives the exciter winding, is injected into one of the coil body flanges that define the exciter winding on its face end, the injection being done in such a way that the stanchions protrude radially from the flange circumference. In an exciter winding wound in multiple layers, the windings that wrap around the stanchions are then located in the uppermost layer in the end region of the exciter winding.
Since the magnet coil is typically surrounded by a housing, in an advantageous embodiment of the invention an opening congruent with the stanchion strip is present in the housing. Through this opening in the housing, access to the radially oriented stanchions on the flange of the coil body is possible. After the calibration operation by welding an appropriate stanchion to the winding portion wrapped around it, the housing opening is closed with a potting composition, preferably epoxy resin.